I think the main issue with Platform 0 is that I believe it was meant to have a dedicated track onto the Buxton line as part of the 2004 modernisation but this was curtailed due to budget cuts. The platform was then a white elephant until it came into use mid timetable in 2008 iirc.
The current signalling system makes it difficult to have a train in 0 and 1 at the same time (the quickest I’ve seen is one departing as the other is arriving) even if one train is right behind the other.
Plus I believe Avanti decided to cut the dedicated member of staff at certain hours for Platform 0, as it wasn’t operationally necessary.
It wasn't budget cuts, it was the realisation that the proposed signalling system would have had difficulty coping with the layout and/or timetable.
Under the proposed layout, Platform 1 would have become the Up Fast, 2 the Down Fast. 3 and 4 the Slows. This ironed out the crossing movements at Slade Lane Junction, but of course the Up platforms at Levenshulme and Heaton Chapel are now on the Fast, where you want to run a London every 20 minutes, plus XC, TPE, etc. Crossing stopping Buxton trains (and potentially anything else going via the Hope Valley) over to the Slow at Heaton Norris Junction to get out of the way means crossing back over at Edgeley Junction, so Platform 0 was the alternative, to allow Buxton/Hope Valley services to be recessed without crossing the layout twice.
After completion, it wasn't brought into regular use, basically it's not really needed in normal working, it 'costs' an extra staff member for dispatch, and transferring between it and the rest of the station takes time (especially once ticket gates went in as P0 is outside the gateline).
I heard a rumour the local council (who may have part-funded it) eventually asked awkward questions, as a result of which NR issued instructions to use it for everything turning left at Edgeley.